Geological Survey Professional Paper 296 Geology of Glacier National Park And the Flathead Region, Northwestern Montana

STRATIGRAPHY (continued)

MESOZOIC ROCKS

JURASSIC ROCKS

The only beds of Jurassic age known are thought to belong to the Ellis group (Middle and Upper Jurassic). If the Morrison formation exists, it was not recognized during the fieldwork and may have been grouped with the Kootenai formation on the map. The distribution of the Ellis is similar to that of the Hannan limestone just described, but the Ellis generally occurs as narrow, inconspicuous bands, which are masked in places by hill wash and talus, in strong contrast to the great cliffed ridges formed by the Hannan. The Ellis consists of soft fissile nearly black shale interbedded with more resistant beds of sandy limestone that weather in conspicuous shades of yellowish brown and dark yellowish orange (fig. 18). In places the basal beds are yellow or yellowish-orange quartzitic sandstone. As a result of faulting, few of the exposures seen in 1948 contain more than a couple of hundred feet of beds, and most of them contain less; but the Ellis group is nearly 400 feet thick in the Saypo quadrangle (Deiss, 1943b, p. 1136-1140), and in the area here discussed the thickness is probably even greater. At the lower end of Swift Reservoir (NE 1/4 sec. 27 and SW1/4 sec. 26, T. 28 N., R. 10 W.) and other nearby areas, the thickness is reported to be 480 feet (Imlay, 1948).

FIGURE 18.—Contorted shale and limestone of the Ellis group on Badger Creek near the mouth of Red Poacher Creek, Maclas Pass quadrangle, Flathead region. The exposure is in a small mass overridden by a large block of Hannan limestone.

A fossil collection made from the Ellis group near the head of Crazy Creek by Harold Masursky proved when examined by Ralph W. Imlay to consist of fragments of Belemnites and plant stems indicative of the Swift formation, the uppermost formation in the Ellis. A collection made by Masursky through a vertical range of 20 feet about 300 yards downstream from the first one is reported by Imlay to contain Pleuromya subcompressa (Meek), Pleuromya obtusiprorata (McLearn), Grammatodon? sp., and ammonite fragments. Imlay regards these fossils as probably from the Sawtooth formation because of the presence of Pleuromya obtusiprorata (McLearn). The Sawtooth is the lowest of the three units into which the Ellis group is divided in this region. The middle one of the three, called the Rierdon formation, is probably represented by a collection made by C. P. Ross at the head of Canyon Creek. Imlay reports that this collection contains Trigonia sp., a belemnite fragment, Meleagrinella sp., Gryphaea nebrascensis (Meek and Hayden), and Camptonectes sp. A collection made by C. P. Ross in the west bank of Badger Creek above the mouth of South Badger Creek (fig. 18) was found by Imlay to contain Gryphaea impressimarginata (McLearn) and an ammonite, Gowericeras. According to Imlay, the first named has been found in abundance elsewhere in western Montana in the upper part of the Sawtooth and lower part of the overlying Rierdon formation, but not in association with Gowericeras, which commonly occurs at a slightly higher position. The collection came from a narrow, much deformed fault sliver, and the ammonite may have been moved from its normal position as a result of the deformation. It is possible, also, that Gowericeras has a longer range than previously known.

While more complete stratigraphic studies are needed, the collections listed above serve to indicate that the subdivisions of the Ellis group that have been recognized (Imlay, 1945, 1948; Cobban, 1945) in nearby areas are present in the region here described. It is impractical on the basis of the present incomplete information to subdivide the Ellis group on the map herewith. Many of the exposures are too narrow to show subdivisions on a map of the scale of plate 2 even if full information was at hand.

CRETACEOUS ROCKS

Kootenai Formation

Several narrow exposures of the Kootenai formation (Lower Cretaceous) stretch diagonally across the mountainous southwestern corner of the Heart Butte quadrangle. The widest and most continuous of these is along the mountain front and connects to the northwest with a broad expanse of Cretaceous rocks in the Marias Pass quadrangle. These latter merge westward with a broad band that reaches from the extreme southwest corner of the Heart Butte quadrangle northwest at least as far as Marias Pass and Summit Creek. This band consists mainly of strata belonging to the Kootenai formation and is so designated on plate 2. It is possible that unfolded or infaulted parts of higher Cretaceous units may be present in it and have escaped recognition. On the other hand, beds of Kootenai age are present near the boundary of Glacier National Park northeast of Marias Pass and are presumably exposed farther north as well. Available data in regard to the Cretaceous rocks east of the Lewis overthrust are not sufficiently complete to make it practicable to show subdivisions on plate 1.

The Kootenai formation, of continental origin, is made up principally of red-purple, reddish, gray, and green sandstone and sandy shale. Some of the shale is carbonaceous and is brown to nearly black. The carbonaceous shale is especially plentiful near the South Fork of Two Medicine Creek. Tiny fragments of black chert are fairly plentiful in the sandstone. Some of the sandstone beds contain numerous cherty and limonitic concretions that weather dark brown. Some of these are fully 2 feet long. In other places the concretions are arenaceous and calcareous. Coarse grit and fine conglomerate are present locally but are nowhere conspicuous. Near Cox and Winter Creeks, and less conspicuously east of Strawberry Creek, several beds of fine-grained limestone occur. These contain fragments of fossils, but identifiable material was not obtained. In other localities, where limestone is absent, some of the sandstone is calcareous. Deiss' (1943b, p. 1143) estimate of maximum thickness of about 1,500 feet for the Kootenai in the northern part of the Saypo quadrangle is probably applicable to the southern part of the Marias Pass quadrangle also. The thickness farther east appears to be somewhat less, which is in accord with Stebinger's (1918, p. 156) estimate of approximately 900 feet for the Kootenai formation in the plains. Direct measurement of thickness in this soft, highly deformed formation would, at best, be subject to a large margin of error.

Colorado Shale

No Cretaceous beds younger than the Kootenai were recognized in the mountains of the Flathead region. If any are present they are mapped with the Kootenai. In the plains east of the mountains, however, many Upper Cretaceous strata are known as a result of Stebinger's work, from which most of the data given below are abstracted. He has mapped several masses of Colorado shale immediately east of the mountains in the Heart Butte quadrangle. Beneath the overburden, these masses probably form a nearly continuous band, which is broken and offset in places by faults.

The basal part of the formation, known as the Blackleaf sandy member, is reported to be present but was not mapped separately by Stebinger within the area of plate 2. It consists of grayish black shale, in part arenaceous, with intercalated dark-gray to black sandstone. Deiss (1943a, p. 238-240; 1943b, p. 1144-1145) has described this member in the Saypo quadrangle in some detail. Some of the upper part of the Colorado shale is also present there. This upper part is generally composed of soft, somewhat fissile, brownish- and grayish-black shale, with subordinate sandstone. Some limestone is included. The Colorado shale, according to Stebinger, has a total thickness in excess of 1,800 feet, but all of this may not be present within the area of plate 2. Unlike the underlying Kootenai formation, the Colorado shale is of marine origin.

Montana Group

Most of the bedrock in the plains of the Heart Butte quadrangle belongs to the Montana group. These rocks extend into the northeastern corner of the Marias Pass quadrangle also, but in most places they are masked by detritus, which is mainly of glacial origin.

Close to the mountains in the Heart Butte quadrangle, Stebinger has distinguished, in ascending order, the Two Medicine formation, Bearpaw shale, and Horsethief sandstone; but in the southeastern part of the quadrangle, he maps the group as undifferentiated. He says that in many places facies changes make the component formations unrecognizable. In and near the northeast corner of the Marias Pass quadrangle, small areas underlain by Virgelle sandstone near the base of the Montana group are distinguished on unpublished maps prepared by Stebinger; and these are shown on plate 2.

As the Montana group was not studied during the 1948-50 fieldwork nothing can be added to published descriptions of its character. In general, the group consists of dark-gray and greenish sandstone and shale beds of marine origin, many of which weather brownish. Fossils are plentiful in some beds. The Virgelle sandstone includes beds in which magnetite is so abundant that Stebinger (1918, p. 165, 1912, p. 329-337) speaks of it as a low-grade, siliceous iron ore. The thickness of the group differs from place to place but is approximately 3,000 feet.

St. Mary River Formation

A few exposures of the St. Mary River formation, which here constitutes the uppermost Cretaceous unit, are shown on plate 2 on the basis of Stebinger's published and unpublished work. The St. Mary River formation is present within the area of plate 1 but is not distinguished on that map. The formation consists, according to Stebinger (1912, p. 330-332), of light-gray fresh and brackish-water clay and sandstone with some red and varigated shale in the upper part. The total thickness of the formation is somewhat less than 1,000 feet. The formation in Canada north of the area of plate 1 has recently been described in some detail (Williams, 1951).

Undifferentiated Cretaceous Deposits

Along the eastern and southeastern borders of Glacier National Park numerous outcrops of beds of Cretaceous age project through the cover of material of Cenozoic age; the Cenozoic deposits are largely glacial. The Cretaceous rocks include all of the formations mentioned above, but the formations are not distinguished on the maps of the present report because of inadequate information.

In the Blacktail Hills and from there northeastward for several miles in the vicinity of the trace of the Lewis overthrust, beds of the Kootenai formation are abundantly exposed. From near Glacier Park Station northward along the mountain border, the Colorado shale is exposed in numerous places and probably constitutes the principal formation present. Near the international boundary in the general vicinity of Belly River, exposures of the lower part of the Montana group were recognized by members of M. R. Campbell's field parties. Farther east in the greatly faulted rocks in and east of the northeast part of plate 1, higher units of the Montana group and also the St. Mary River formation are present.